Background A study was undertaken to assess the impact of air quality warnings associated with ground-level ozone on outdoor activities in Southern California.

Methods Data on aggregate daily attendance at two major outdoor facilities were collected and merged with observed and forecasted air quality and meteorology at the daily level. A quasi-experimental regression discontinuity design was used to estimate the impact of warnings.

Results Attendance declined significantly when stage 1 air quality warnings (‘smog alerts’) were issued. Consistent with expectations, responses were greater for populations more likely to be considered susceptible and more likely to be local residents.

Conclusions Air quality warnings are an important policy tool for protecting the public's health from high levels of ambient air pollution.